1. Field of the Invention
The present invention relates to a kneading screw and an extruder provided with the kneading screw.
2. Description of the Related Art
Generally, a compound resin material such as a plastic compound is produced by supplying pellets of a polymeric resin as a matrix material and a powdered additive into a barrel of an extruder and feeding them to a downstream side while kneading them with a kneading screw inserted into the barrel. The kneading screw includes an axial combination of plural segments of different types to axially form sections which exhibit different functions. As examples of such sections exhibiting different functions, there are a feed section for feeding materials to the downstream side, and a kneading section for kneading the materials.
As segments which constitute the kneading section, there are used such kneading segments as a rotor segment and a kneading disc segment. These kneading segments are provided with kneading flights, whereby a large shear force is imparted to materials, making it possible to knead the materials. However, also from the materials, a large force as a reaction force against the shear force is apt to be exerted on the kneading flights.
If the force applied to the kneading screw through the kneading flights is large, there is the possibility that the kneading screw will rotate in a deflected state. In this case, ends of the kneading flights come into contact with the inner wall of the barrel and damage called wear is apt to occur. For preventing the occurrence of such wear it has been considered effective to form a large tip clearance between the barrel and the kneading flights.
For example, in Japanese Patent Laid-Open Publication No. Hei 9 (1997)-117954 there is disclosed a kneading screw wherein a rotating outside diameter of each kneading flight in the kneading section is set at 0.95-0.98 relative to the other sections than the kneading section and is thus made larger in tip clearance than in the other sections.
In Japanese Patent Laid-Open Publication No. 2000-296517 there is disclosed a kneading screw wherein two types of kneading disc segments different in rotating outside diameter are combined axially alternately although the purpose of the combination is to decrease the number of gels formed.
In the patent laid-open publications Hei 9 (1997)-117954 and 2000-296517, as a result of setting small the rotating outside diameter of the kneading screw in the kneading section relative to the other sections than the kneading section for the purpose of avoiding the occurrence of wear, there is formed a large difference in height in the radial direction between an upstream side and a downstream side of the kneading section in the kneading screw. If the kneading screw has such a difference in height, stress is apt to be concentrated in the portion where the difference in height exists, thus resulting in that bending or breakage of the kneading screw becomes easier to occur.
In this connection, in the kneading screw disclosed in the patent laid-open publication Hei 9 (1997)-117954, a lower limit is set for the rotating outside diameter to prevent the stress on a spline shaft from becoming large. However, with such a small difference in height, there still occurs wear, with a consequent possibility that there may occur abnormal wear of kneading flights.
Moreover, the stress which occurs in the portion where the difference in height is formed, i.e., the stepped portion, acts on the spline shaft repeatedly with rotation of the kneading screw. Therefore, even if the force exerted on the stepped portion is of a magnitude not causing breakage, a repeated long-term exertion thereof on the stepped portion is likely to cause fatigue fracture and breaking of the spline shaft.